The present invention concerns forming of a paper or board web from aqueous wood fibre stock. More specifically, the invention concerns a method and device for forming paper or board at a high speed in the early stage of web formation.
When making paper of aqueous wood fibre stock, the initial formation was then done on one forming wire, such as a Fourdrinier wire part, or in a twin-wire former, such as the so-called gap former, wherein a pair of opposite wire loops traveling in the same direction forms a closing gap, into which a stock jet is supplied from a headbox into the space between the forming wires, water is removed from the stock through the forming wires in order to start formation of the paper web by leaving the woodpulp fibres randomly distributed on the forming wire or in between the forming wires traveling together.
Depending on the quality of the paper or board to be made, fibre pulps of different types are used. The quantity, with which water can be removed from different fibre pulps in order to bring about a paper product of good quality, is a function of many factors, such as, for example, a function of the desired standard of the paper product, of the desired caliper of the paper product to be made, of the design velocity of the paper machine, and of the desired standard of fines, fibres and fillers in the final paper product.
It is known in the state of the art to use forming shoes to guide one or two forming wires on the forming section of the paper machine. It is also known to use a so-called forming roll equipped with an open, for example, perforated surface to receive water through the forming wire into the interior of the forming roll from the fibre pulp supported by the outer surface of the forming wire.
It is further known to use a forming shoe, whose surface has grooves starting in the downfeed direction from the leading edge of the forming shoe and extending at a small angle in relation to the machine direction (that is, in relation to the traveling direction of the paper web through the paper machine).
Devices of several types are known in the paper machine's forming section, that is, in the former, such as foil blades, suction boxes, hitch rolls, suction rolls and rolls provided with an open surface, which have been used in several different formations and sequences when trying to optimize the quantity of exiting water, the time and the location during the paper web formation. The making of paper is still an art in part in that simply removing water as quickly as possible will not produce a paper product of optimum quality. In other words, the production of a high-quality paper product at high velocities, for example, at approximately 2000 m/min, is a function of the quantity of removed water, of the manner in which water is removed, of the duration of dewatering and of the location where water is removed from the stock or in between the forming wires.
Earlier when paper machines operated at lower velocities, for example at 900-1200 m/min, relative utilization of the above-mentioned factors could vary in order to achieve the desired quality in the paper product. In addition, when desiring to maintain or improve the product quality when making a product at higher speeds, unforeseen problems will occur in most processes, so that either the production quantity must be reduced to maintain or achieve the desired quality or the desired quality must be sacrificed in order to achieve a higher production quantity.
The blade elements or foils of earlier forming shoes or blade shoes had a forming shoe surface of a curved or planar shape, they had several gaps in between the blade elements, which extended in the longitudinal direction over the blade element length. The gaps for their part define leading edges for the blade elements, which blade elements are arranged in the cross-machine direction at right angles to the traveling direction of the forming wire. Such an arrangement works well. The stock jet is directed against the forming wire over the leading edge of the forming shoe/blade in such a way that a part of the water in the stock jet will travel through the forming wire and end up below the shoe/blade. Each foil, blade element or forming shoe is either open to atmospheric pressure at its bottom or they are connected to an underpressure source in order to improve dewatering by forcing water into gaps in between adjacent foils or blade elements. The blade elements form the top surface or deck of the foil or forming shoe.
However, with increasing paper machine velocities to make paper products with ever improved economy, new phenomena begin occurring in connection with the paper machine's runnability and also relating to the appearance and internal structure of the produced paper product. Most of these changes are not desirable.
These phenomena may occur in different forms, such as an undesirable distribution of fines and fillers in the paper product's surface or internal parts, whereby the acceptable retention or finer retention would decrease. These changes and imperfections are disastrous for the paper product and affect its saleability.
There are two techniques in principle, which are in general use in the formation of printing stock and writing paper, that is, blade type gap formers and roll gap formers. Both these techniques have certain advantages and disadvantages, of which the following may be listed.
Advantages of the roll gap former are that the impingement of the headbox jet onto a roll having a relatively large radius is very insensitive to minor geometrical errors in the jet quality and to external effects, such as windage and water drops, that Z direction properties, such as regards fillers and anisotropy, can be achieved and excellent two-sidedness due to the fact that a fibre mat is formed at first at the same time on both wires at a constant (that is, non-pulsating) dewatering pressure, and that a good retention can be achieved due to the fact that initially a constant (that is, non-pulsating) dewatering pressure exists in the dewatering zone. A considerable disadvantage of this technique is that rotation of the forming roll results in a vacuum pulse on the exit side of the roll nip. This pulse will partly damage (crush) the formed paper structure as it travels from the zone with a constant pressure into the following zone with a pulsating pressure, if the paper is too wet at this point. In practice, this limits the formation quality of this type of former, because the quantity of water, which can be made to transfer into the pulsating dewatering zone, is limited by this vacuum pulse. Essential disadvantages are also the costs of the forming roll and its spare parts as well as the roll's need of maintenance and the resulting time of machine shutdown. Another noticed problem with the roll gap former is the insufficient dewatering capacity at high speeds (>1600 m/min) and with dense pulps.
Advantages of the blade type gap former are that because to begin with the jet dewatering is carried out at a pulsating pressure, the formation potential of this type of former is very good. Since all dewatering components are fixed, acquisition and maintenance costs are lower than when using a roll as the first dewatering device.
This technique has the following disadvantages, among others. The jet inpingement onto a shoe having a relatively large radius and constructed to create pulsating dewatering is very sensitive to numerous errors. This is the main limitation of an efficient operation of formers of this type. The initial dewatering is quite asymmetric, which results in a very one-sided paper structure in the Z direction, especially as regards fillers and anisotropy. Because dewatering of the pulp is initially done with a pulsating pressure, the retention is low.
As regards the state of the art, reference is also made to U.S. Pat. No. 5,798,024; US patent application publication No. 2001/0025697, now U.S. Pat. No. 6,372,091; and GB patent No. 1,288,277.